The invention relates to broad-band antennas and ground planes therefor.
In designing broad-band antennas, such as used for communication purposes, radar surveillance, and in general in any application where there is a need to receive signals over a bandwidth of an octave or more, there is a limited number of basic antenna configurations that are suitable. The most notable are: (1) log periodic, and (2) spiral types. These antenna configurations are multimode and may achieve a bandwidth ratio of 100:1. Both types (log periodic and spiral) may have been used in combination with various ground plane shapes and designs; however, the inventors herein have found that existing ground plane configurations unduly limit the performance that is theoretically predicted for log periodic and spiral antennas.
In general, the purpose of the ground plane is to efficiently redirect one axis of radiation from the log periodic or spiral antenna elements. When these antenna structures are excited, without the provision of a ground plane, the energy is radiated bidirectionally along the axis of the antenna away from its center; however, the radiation pattern in most applications is useful in one direction only and the radiation in the undesired direction must be either absorbed, thereby reducing efficiency, or reflected in the desired direction. The ground plane serves the latter purpose by first blocking off the radiation in the unwanted direction and, furthermore, causes that energy to be reflected back through the excited or driven antenna element and outwardly along the desired axis of radiation, reinforcing the energy that propagates directly (without reflection) from the excited antenna element. However, commonly used ground plane configurations and fabricating materials are believed to either restrict the otherwise theoretically attainable bandwidth, or to develop such large resistive losses that the overall antenna efficiency is drastically reduced.
To understand the limitations of existing antenna configurations, it is helpful to list some important characteristics of spiral and log periodic antennas. In such antenna configurations, the radially innermost area of the antenna is active at the higher frequencies of the antenna bandwidth and the active region of the antenna progressively advances outwardly and radially as the frequency decreases and the wavelength increases, to the outermost perimeter of the antenna, at which the antenna is active at the lowermost frequencies of the antenna bandwidth. In this sense, the spiral and log periodic antennas are frequency-independent, or as is sometimes said, frequency-repeating in that the electrical properties of the antenna repeat at radially increasing circles as the frequency decreases, each frequency level exciting a different annular region of the antenna.
One prior art ground plane configuration is a flat conductive metal plate arranged in combination with a flat planar, spiral radiating antenna element situated approximately one-quarter of a wavelength above the flat ground plate. The shortcoming of a flat ground plate in combination with a planar spiral radiating element is that the electrical spacing between the spiral and the ground plate varies with frequency but the physical spacing stays constant, thereby placing a constraint on the bandwidth of the device, typically limiting the bandwidth to about a 3:1 frequency ratio.
A number of configurations have been proposed to remove this particular bandwidth constraint. One effort to broad-band the ground plane is to form it in the shape of a cone in which the physical spacing, between the planar spiral element and the ground cone surface of which the radiation is reflected, increases in the radially outward direction so that the electrical spacing, which also increases radially outward from the center of the spiral, maintains an approximately constant one-quarter of the wavelength relationship at all radial locations.
However, this conical "ground plane," when configured with a regular, smooth conical surface, has been found by the inventors herein to generate or sustain undesirable modes of excitation which either diminish or destroy certain other and desired antenna modes that are associated with optimum radiation patterns. In particular, it is believed that such undesirable excitation modes result from rearward radiation from the driven antenna element being reflected at abnormal or unpredictable angles from the cone surface which reexcite different regions of the antenna in a nonconstructive manner. Although these destructive reflections are not fully understood, it is believed that they are due, in part, to excessive radial currents that are not present in a flat ground plane, which suppresses such radial currents because of the close and parallel spacing between the flat ground plane surface and the flat plane of the driven elements. In the conical ground plane, due to the increasingly greater physical spacing between the cone surface and the planar radiating elements, it is thought that the radial currents are not sufficiently suppressed and become excessive. The undesired reexcitation modes are normally at higher orders than those modes that produce the initial or primary radiation. Previous attempts to solve these excessive radial currents, such as by use of resistive radial fins on the cone structure and/or embedding a spiral antenna in a spiral cavity, have not met with success, either because of loss of efficiency or practical limitations due to the proposed antenna geometries.
In regard to the above discussion of "ground planes," it is noted that the term "plane" is a misnomer when used to refer to conical and other nonplanar ground or reflector configurations; however, the term "ground plane" is accepted as a term of art meaning an antenna structure or component that electrically serves to reflect wave energy in a way analogous to a conventional, flat ground plane, and is used herein to convey this broader functional meaning.
Not all prior art attempts to broad-band the directional antennas of the above type have involved the use of reflective ground planes. In one proposed configuration, the radiating spiral or multiarm spiral is shaped itself in a conical configuration. For acceptable unidirectionality of the radiating pattern, the spiral antenna cone must have an overall length of a wavelength or more, and hence the shape of the resulting structure does not lend itself to flush mounting applications. In this regard, it is frequently desirable in using these broad-band multiarm log periodic and spiral antennas, to mount the plane of such antennas flush with the surface of an aircraft and hence, when the antenna shape is reconfigured for its electrical properties, the resulting geometry is not always optimum for flush mounting.
Another disadvantage of the conical spiral is that the electrical phase center moves axially along the cone, causing severe defocusing that reduces the antenna gain when used to feed a parabolic reflector. Furthermore, when the conical spiral is excited in multiple modes, the phase center for each mode has a different axial position which again creates defocusing losses different for each mode.